Can you please fill out the sticky questions and post a picture?

 

Housing
What size is your tank? - 25 gallon
What temperature is your tank? 78 degrees
Does your tank have a filter? - yes
Does your tank have an air stone or other type of aeration? - no
Is your tank heated? - yes
What tank mates does your betta fish live with? - pleco

Food
What type of food do you feed your betta fish? - ward lets Betta Pellets
How often do you feed your betta fish? - 1-2 times daily But he hasn't been eating

Maintenance
How often do you perform a water change? - twice a month
What percentage of the water do you change when you perform a water change? - 50%
What type of additives do you add to the water when you perform a water change? - nutrafin water conditioner

Water Parameters:
Have you tested your water? If so, what are the following parameters? - No

Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

Symptoms and Treatment
he hasn't been eating
He hasn't been active
He is pineconing
He isn't bloated
He isn't changing colours
I started noticing this today
I have not begun any treatment. I will tomorrow
Picture;

 

Cosmo had similar symptoms a few weeks ago. He just stayed in one spot for ages and I got really worried. He wouldn't eat either. I moved him to hospital and realised he was pine-coning too. His belly was only a tiny bit grey and swollen. The next day I noticed his poop was white and stringy and put it down to internal parasites. So I immediately started him on epsom salt (1tsp/gal), IAL and API Erythromycin. I also raised the temperature and did 50% water changes every other day. I am not sure if it is what others would do, but it worked. Good luck!

Also I would like to add that those pellets aren't good for him. I think they're full of fillers? You need to get a good quality pellet that has meat as the first ingredient, such as Omega One Betta Buffet Pellets, New Life Spectrum (NLS) Betta Formula or Ocean's Nutrition Atison's betta food/pro. And I would use Seachem Prime (highly recommended) or Stress Coat Plus for a water conditioner. You should also be doing a water change more often. Like 10-20% once or twice a week is how much I do, but others recommend about 50%.

A picture would help us help you..

 

+1 to iSheree.

It would also help if you gave us actual numbers for the water parameters, most of the time bad water quality is the reason behind illnesses in fish.

As for the water changes, no matter how big the tank you need tot be doing at least one water change per week. You'd be better off doing 30% changes weekly instead of your current schedule.

How big is the pleco?